Most prior attempts to create color images of photographic quality using the science of electrophotography have employed liquid developers. For many years it was thought that liquid developers were the only developers with fine enough particles to give the resolution ordinarily experienced with silver halide photography. Recently, multicolor images have been formed using toner particles finer than 8 microns in diameter and in some instances finer than 3.5 microns in diameter. With such size particles granularity comparable to silver halide photography is obtainable.
Finishing color images with such fine particles while maintaining resolution has posed many problems. Ordinary heated roller, pressure fusing has a tendency to spread the particles on the surface of a receiving sheet, destroying the fine granularity created by the fine particles. Infrared heating also causes some spread of the particles as the particles are encouraged to flow in order to become fixed.
Of more concern, the particles are formed on the surface of the receiving sheet in a series of layers, the height of which is dependent upon the density and the particular combination of colors needed to make up the image. This creates a substantial relief image which is quite noticeable to the eye. This is especially the case after infrared fusing, but also is apparent after hot pressure roller fusing of the type used in most copiers. This relief image is sufficiently unacceptable that a multicolor print made with it would not be competitive with a comparable silver halide product.
In most photographic work a glossy appearance is desirable and provides an appearance of image sharpness. However, with prior copying fusing systems, gloss levels in excess of 20 are rare. Further, the same variation in amount of toner which causes relief also causes a variation in image gloss across a single image.
U.S. Pat. No. 4,337,303, Sahyun et al, issued Jun. 29, 1982, discloses a relatively low speed method of transferring fine toner particles from a photoconductor to a receiving sheet having a thermoplastic coating on it. According to that patent the thermoplastic coating is heated to its softening point, preferably a temperature between 20.degree. and 70.degree. C. Under moderate pressure the toner is "encapsulated" in the thermoplastic layer, with less than 25% of the particles protruding.
Japanese Kokai 63-92965 (1988), laid-open Apr. 23, 1988, discloses a method of treating a color image on a thermoplastic layer on a receiving sheet by passing the sheet between a pair of rollers, with at least the roller contacting the image being heated in the presence of a pressure of 4 kg/cm.sup.2. Both rollers are formed of silicone rubbers. It is suggested that, if the thermoplastic is heated higher than its softening point but lower than the softening point of the toner, the toner can be pushed into the thermoplastic. This procedure, it is suggested, will remove the unevenness of the surface of the electrophotographic image.
U.S. Pat. No. 4,780,742 shows a method and apparatus for treating a fixed color toner image carried on a transparency sheet. The sheet is passed between a thin plastic sheet and a pair of rollers in the presence of heat which presses the thin sheet around the toner to soften, fuse and add gloss to the image. The thin sheet is peeled off after the image has cooled. According to the patent, this provides an image that scatters light less in projection.
European patent application 0 301 585 published Feb. 1, 1989, shows a glazing sheet used to increase the gloss of either a toner image on a paper support or a dye and developer in a thermoplastic coating. The glazing sheet is pressed against the paper sheets with moderate pressure and the dye-thermoplastic sheets with substantial pressure. Resolution, relief and variable glossing are not mentioned as problems.
In the latter two references the image and sheet are allowed to cool before separation. This approach to preventing release in pressure fixing devices is shown in a large number of references; see, for example, European patent application 0 295 901 and U.S. Pat. No. 3,948,215.
In general, the use of a softenable outer layer into which the toner is imbedded permits finer resolution and reduces toner image relief. However, high pressure in embedding such toner can cause irregularities in the thermoplastic surface which also detract from the appearance of the image.
U.S. Pat. No. 4,419,004 issued Dec. 6, 1983 to Kuehnle shows an electrophoretic transfer and fixing method in which a toner image carried in a liquid is transferred under the urging of an electric field and embedded in a heat-softened overcoat on a receiving paper. An electrophoretic migration apparently begins in the liquid developer and continues in the molten overcoat.